Add janet_vm_save and janet_vm_load.

This lets a user multiplex multiple Janet VMs on a single
thread or process if they are willing to implement context switching
themselves.

meson.build

@@ -127,6 +127,7 @@ core_src = [
   'src/core/regalloc.c',
   'src/core/run.c',
   'src/core/specials.c',
+  'src/core/state.c',
   'src/core/string.c',
   'src/core/strtod.c',
   'src/core/struct.c',